The present invention relates to an inerting method for reducing the risk of, and for extinguishing, fires in enclosed spaces, and to apparatus for carrying out this method.
In the case of enclosed spaces into which human beings or animals enter only occasionally, and installations which are impacted adversely by water, it is known to lessen the risk of fires by reducing the oxygen concentration in the area in question to an average value of about 12%. Given this oxygen concentration, most combustible materials can no longer burn. The areas concerned are mainly data processing areas, electric switch and distribution rooms, enclosed installations and storage areas containing high-grade valuable goods.
The extinguishing effect resulting from this method is based on the principle of oxygen expulsion. It is known that the normal ambient air consists of 21% oxygen, 78% nitrogen and 1% other gases. For fire extinction, the nitrogen concentration in the space concerned may be further increased by introducing pure nitrogen so as to reduce the oxygen portion. It is known that an extinguishing effect commences once the oxygen content falls under 15% by volume. Depending upon the combustible materials in the space concerned, it may be required to further reduce the oxygen content to the mentioned 12% by volume.
With said xe2x80x9cinert gas extinguishing techniquexe2x80x9d, as the flooding of a fire hazardous or burning space with oxygen-expulsion gases such as carbon dioxide, nitrogen, rare gases and mixtures thereof is called, the oxygen-expulsion gases are usually stored in a compressed manner in steel cylinders in specific side rooms. In the case of need, the gas is then conducted into the space in question by means of piping systems and corresponding exit nozzles. Fire extinction by means of the inert gas extinguishing technique, however, encounters certain problems and has clear limits in view of the size of the space. Large spaces having, for instance, a basic area of 20xc3x9750 m and a 6.5 m height result in a volume of 6,500 m3. In accordance with the known standards, the steel cylinders used are those having a volumetric capacity of 80 l. Inert gas extinction facilities are filled with a pressure of 200 bar, which is presently the upper standard parameter due to the ultimate loading capacity of the available armatures. With a cylinder pressure of 200 bar, an 80 l, cylinder for example, holds 18.3 kg of nitrogen resulting in 16 m3 nitrogen in the relaxed state at 1 bar ambient pressure. In order to flood the aforementioned space having a volume of 6.500 m3 with inert gas, the contents of about 300 steel cylinders would be required. In a filled state, such a cylinder has a weight of about 100 kg, which, given 300 cylinders, would result in a weight of 30 tons.
In addition there would be the weight of the pipes and armatures, so that very high demands would have to be made on the load ability of the store rooms. Moreover, a large floor space would be required for such a number of cylinders. Thus, it is evident that the inert gas extinction technique in connection with larger spaces encounters problems in view of the storability and the carrying capacity of the store rooms. To store the cylinders in a cellar is not a satisfying solution either, although the carrying capacity there is not of importance. Long conduits would have to be laid from the cellar to the upper floors involving additional construction labor, which frequently cannot be coped with later, and moreover prolongs the flow-in time of the inert gas, in an inappropriate manner.
It is an object of the present invention, therefore, to provide an inerting method for reducing the risk of fires and for extinguishing fires in enclosed spaces, allowing an effective extinction of a fire while keeping the storing volume of the inert gas cylinders at a minimum.
Said object is provided by means of an inerting method of the above-mentioned kind comprising the following steps: at first, the oxygen content in the enclosed space is reduced to a selected base inerting level of, for example, 16%, and in the event of a fire, the oxygen content is further reduced to a selected complete inerting level of, for example, 12% by volume or less. A base inerting level of an oxygen concentration of 16% by volume does not entail any risk for persons or animals, so that they can still enter the space without any problems. The complete inerting level can either be adjusted at night when no persons or animals are likely to enter the space in question, or directly in response to a detected fire. With an oxygen concentration of 12% by volume, the flammability of most materials has already been sufficiently reduced so that they can no longer start to burn.
The present method is particularly advantaged in that the number of containers for oxygen-expulsion inert gases required in the event of a fire is clearly reduced. Thus, the total costs of the fire prevention and extinction system is considerably reduced. Furthermore, from a constructional aspect, a smaller pressure relief facility for the space is required, because in the event of a fire, only a smaller gas volume has to flow in during the short time available, for which a constructional relief facility has to be provided.